The present invention relates to a method of recovering metals from sludges or the like deposits containing sparingly soluble metal compounds such as metal hydroxides formed in the course of treatments of industrial effluents.
Hitherto, industrial effluents discharged in the step of the surface treatments of metals or the electrolytic refinement of metals were usually neutralized with an alkali to precipitate various kinds of metal ions constituting a source of pollution as sparingly soluble metal compounds such as metal hydroxides and carbonates, and then the precipitate was separated as sludge by floatation or deposition. As a complete treatment of the resulting sludge for preventing any secondary pollution is difficult, the prior art method of disposing the sludges in such circumstance comprises concentrating the sludges by dehydration, fixing the concentrates with concrete and then using it for reclamation.
With a view to attaining effective re-utilization of limited expensive metal resources and prevention of environmental pollution, the present inventors have made researches for effectively recovering metals directly from sludges containing sparingly soluble metal compounds in the form of metal hydroxides, oxides, salts, etc. released in the treatment of industrial effluents. The present invention is based on such researches.